The present invention relates to a delayed AGC circuit used in a high-frequency signal receiver such as a television receiver having a high-frequency amplification stage and an intermediate frequency amplification stage, or more in particular to a delayed AGC circuit suitable for a semiconductor integrated circuit construction.
The gain of the high-frequency amplification stage (hereinafter referred to as an RF amplification stage) and the video intermediate frequency amplification stage (hereinafter referred to as a PIF amplification stage) of the television receiver is controlled by automatic gain control (hereinafter referred to as AGC) voltage changing with the antenna input electric field, i.e., the video detection output so that a uniform video detection output may be provided even when the antenna input electric field undergoes a change.
The delayed AGC circuit has first and second negative feedback control states. Under the first negative feedback control state, the RF amplification stage is operated at a maximum gain while the PIF amplification stage alone is subjected to automatic gain control from the view points of the noise characteristics and mixing and/or modulation when the antenna input electric field is weaker than a predetermined level, e.g., 70 dB.mu.. Under the second negative feedback control state, on the other hand, the gains of both the RF and PIF amplification stages are controlled in such a manner that the gain of the PIF amplification stage undergoes substantially no change while the change in the gain of the RF amplification stage is dominant when the antenna input electric field is stronger than the abovementioned predetermined level.
The delayed AGC circuit thus comprises a switch for switching its conditions in accordance with whether the AGC voltage has exceeded a predetermined level and an adjustable setting means for setting the predetermined level. By the condition of the switch, switching between the first negative feedback state and the second negative feedback state is accomplished.
An integrated circuit (hereinafter referred to as IC) containing the PIF amplification stage, an AGC detector stage and the switch making up the abovementioned delayed AGC circuit in a single chip is disclosed as AN247P in the Japanese publication "'77 National Semiconductor Handbook" issued on June 30, 1976. This well-known IC AN247P has two terminals connected with input and output terminals of a filter for removing the ripple and a high-frequency component thereof for horizontal and vertical periods from the detection voltage generated by the AGC detection means, and a terminal supplied with a reference voltage for setting AGC voltage level for switch change-over. The two IC terminals connected with the input and output terminals of the filter are connected to the output terminal of the AGC detection stage and the gain control terminal of the IF amplification stage within the IC chip, respectively.
With the increase in the scale of integration within the IC chip, cost is reduced and reliability is improved. In view of this, efforts are being made to increase the scale of integration of the IC for the television receiver. The structural restraint of IC as a component, however, limits the number of IC connecting pins, i.e. the number of IC terminals, which in turn limits the scale of integration of circuits incorporated in an IC chip. As a result, it is necessary to reduce as far as possible the number of the IC terminals required for the circuits contained in the IC chip in order to increase the scale of integrated circuitry.